There's a function in there (named scale) for multiplying a point by a scalar. There's also a function in there (named add) for adding two points together; for point subtraction, you can "flip" one of the arguments (i.e. change the y coordinate to: -y % secp256k1_field_order) before doing the addition (see: How to subtract two points on an elliptic curve?). For adding/subtracting a scalar to/from a point, I can't help you (I don't believe that operation has a useful definition unless you first convert the scalar into a point).